CRAWFORD, Texas (CNN) --A senior Bush administration official told CNN
on
Saturday that additional U.S. military personnel have been dispatched
to Colombia,
where leftist rebels have acknowledged holding three Americans captive.

The official declined to provide a details of how many U.S. military
forces were
in the South American nation but said they were part of "contingency
planning."

The official said any discussions of a possible rescue operation were
premature
and that U.S. authorities were focused on providing intelligence and
other
support to Colombian authorities, who were taking the lead.

White House spokesman Ari Fleischer told reporters here that, "I'm not
going
to describe any additional deployments, but suffice it to say that
we work
closely with Colombia. We have before. We will continue to do so."

Colombia's largest rebel group has said the lives of the three U.S.
citizens are at
risk if the government does not halt military operations in rebel-held
territory.

In a communiqué posted Saturday on its Web site, the Revolutionary
Armed
Forces of Colombia -- known by its Spanish acronym, FARC --
acknowledged for the first time that it had captured three Americans
who
survived a plane crash February 13 in rebel-held territory.

The rebels also claimed responsibility for shooting down the plane,
which they
said had been on a mission to spy on them. Colombian and U.S. military
forces
have blamed the crash on mechanical problems.

"We can only guarantee the life and physical integrity of the three
official gringos
in our power if the Colombian military immediately suspends military
operations
and overflights in the area," said the communiqué, which was
datelined "The
mountains of Colombia, February 21."

The State Department refused to comment on the FARC announcement and
repeated previous statements holding the FARC "responsible for the
safety,
health and well-being" of the Americans.

The United States "demands their safe release," a State Department official
said.

The State Department official said the United States has "not authorized
any
group to negotiate" with FARC for the hostages' release.

Since the crash, more than 2,000 Colombian soldiers have been scouring
the
rugged terrain in the southern part of the country in search of the
Americans,
whom the communiqué identified as CIA agents.

U.S. planes, including AWACS surveillance craft, have been flying overhead
to
help direct the search for the men, who were among five people -- four
from
the United States and one Colombian -- aboard the Cessna 208, which
was
contracted by the U.S. Defense Department. The men were private citizens
under contract with the DOD, U.S. officials have said.

The Colombian army said rescuers reached the site within half an hour
of the
crash and found the executed bodies of the other two men -- a Colombian
and
an American -- near the wreckage of the incinerated plane.

The group had been on an intelligence mission en route from the capital
to
Florencia, in Colombia's Caqueta Department, a region known to harbor
FARC guerrillas, the Colombian armed forces said.

-- CNN senior White House correspondent John King and journalist
Asdrubal Garcia contributed to this report.